This is a multidisciplinary study of synaptic transmission between the inner hair cells (IHC) of the mammalian cochlea and the radial afferent neurons of the spiral ganglion. Its purpose is to describe for the first time the quantal release of transmitter by IHCs, to examine the role of calcium in mediating transmitter release and the initiation of postsynaptic action potentials in this system. Another component of the study will be to identify morphological correlates of synaptic transmission by the IHCs under conditions of varying rates of transmitter release by following the incorporation by IHCs of the tracer horseradish peroxidase (HRP) at the ultrastructural level. The physiology of the synapse will be assessed by intracellular recording from the radial afferents near their points of synaptic contact with the OHCs and by recording from single cochlear nerve afferents during simultaneous perfusion of the perilymphatic spaces with solutions containing altered divalent cation content. The number and ultrastructure of active zones supplying radial afferents will be assessed by serially reconstructing their terminations on the hair cells with high voltage electron microscopy (HVEM). The long range goal is to develop a physically-based model of this sensory synapse. The study has public health implications in that it will open the way for the study of drug and disease mechanisms which may specifically affect the synapse.